send RSA public key to iphone and use it to encrypt

Question

I have a TCP socket server and I want to do the following w/o using SSL:

1. On server, make RSA key pair (I know how to do this using openssl's crypto library)
2. On server, send the public key to iphone and keep the private key.
3. On client(iphone), want to encrypt a message using the public key, using SecKeyEncrypt.
4. On server, decrypt the message.

The message is short enough so that the PKCS1 padded result fits into 128 bytes.

I don't know how to do 2~4. Anyone knows?

Answer 1

This should do what you're asking - it encrypts data with the server's public key. It's not subject to MITM attacks, unless the attacker has a copy of your private key and its password (communicating via non-SSL, however, still is, but the data you encrypt with the server's legit public key will be nearly impossible to decrypt).

I cobbled this together from Apple's docs, this site, the Apple developer forums and probably elsewhere. So thanks to everyone I cribbed code from! This code assumes several things:

1. You've already generated your RSA key pairs (I'm using a 4096-bit key and it seems speedy enough) and, using the private key, created a DER-encoded certificate called "cert.cer" that you put in your resource bundle of your app (obviously, you can also download the cert from your server, but then you're open to MITM attacks again). By default, OpenSSL generates a PEM encoded cert, so you have to convert it with "openssl x509 -in cert.pem -inform PEM -out cert.cer -outform DER". iOS will barf on PEM. The reason I use a cert is it's actually easier to work with, and is supported in iOS. Using just the public key isn't (though it can be done).

2. You've added Security.framework to your project and you #import <Security/Security.h>.

/* Returns an NSData of the encrypted text, or nil if encryption was unsuccessful.
Takes the X.509 certificate as NSData (from dataWithContentsOfFile:, for example) */

+(NSData *)encryptString:(NSString *)plainText withX509Certificate:(NSData *)certificate {

    SecCertificateRef cert = SecCertificateCreateWithData(kCFAllocatorDefault, (__bridge CFDataRef)certificate);
    SecPolicyRef policy = SecPolicyCreateBasicX509();
    SecTrustRef trust;
    OSStatus status = SecTrustCreateWithCertificates(cert, policy, &trust);

    SecTrustResultType trustResult;
    if (status == noErr) {
        status = SecTrustEvaluate(trust, &trustResult);
    }

    SecKeyRef publicKey = SecTrustCopyPublicKey(trust);

    const char *plain_text = [plainText UTF8String];
    size_t blockSize = SecKeyGetBlockSize(publicKey);
    NSMutableData *collectedCipherData = [NSMutableData data];

    BOOL success = YES;
    size_t cipherBufferSize = blockSize;
    uint8_t *cipherBuffer = malloc(blockSize);

    int i;
    for (i = 0; i < strlen(plain_text); i += blockSize-11) {
        int j;
        for (j = 0; j < blockSize-11 && plain_text[i+j] != '\0'; ++j) {
            cipherBuffer[j] = plain_text[i+j];
        }

        int result;
        if ((result = SecKeyEncrypt(publicKey, kSecPaddingPKCS1, cipherBuffer, j, cipherBuffer, &cipherBufferSize)) == errSecSuccess) {
            [collectedCipherData appendBytes:cipherBuffer length:cipherBufferSize];
        } else {
            success = NO;
            break;
        }
    }

    /* Free the Security Framework Five! */
    CFRelease(cert);
    CFRelease(policy);
    CFRelease(trust);
    CFRelease(publicKey);
    free(cipherBuffer);

    if (!success) {
        return nil;
    }

    return [NSData dataWithData:collectedCipherData];
}